Tolylene diisocyanate is widely known as versatile isocyanate for use as a raw material for polyurethane, polyurea and the like.
Conventionally, tolylene diisocyanate has been industrially produced by reaction (phosgene method) between diaminotoluene and phosgene. However, phosgene has various problems such as being highly toxic and troublesome in handling, and requiring careful attention to corrosion of the equipment because it by-produces a large amount of hydrochloric acid. Accordingly, there is a need to develop industrial production methods for tolylene diisocyanate that can replace the phosgene method.
Generally known tolylene diisocyanate production methods that do not use phosgene include a method (carbonate method) in which amine is carbamated with dialkyl carbonate and thereafter the obtained carbamate is thermally decomposed, and a method (urea method) in which amine is carbamated with urea, N-unsubstituted carbamic acid ester and the like and thereafter the obtained carbamate is thermally decomposed.
For production of aromatic di- and/or polyisocyanate, for example, a method has been proposed in which primary aromatic di- and/or polyamine and O-alkylcarbamide acid ester are reacted in the presence of urea and alcohol to give aryl-di- and/or polyurethane, and the obtained polyurethane is thermally decomposed.
In particular, for production of tolylene diisocyanate, specifically a method has been proposed in which toluoylene diisocyanate is produced by causing 2,4-diaminotoluol, carbamide acid ethyl ester and ethanol to be reacted to give 2,4-di-(ethoxycarbonylamino)-toluol, and thermally decomposing the obtained 2,4-di-(ethoxycarbonylamino)-toluol (see, for example, Patent Document 1 below).
According to such methods, tolylene diisocyanate can be produced without using phosgene.